1. Field of the Invention
The present invention relates generally to an internal combustion engine valve train and more specifically to a poppet valve control arrangement which features a pair of rocker arms which are arranged in a scissor-like manner and wherein one of the arms is used to lift the valve while the other is used to close the same.
2. Description of the Prior Art
Poppet valves used in internal combustion engines are usually biased to a closed position using a relatively strong spring and moved against the force of the spring by a cam, or a cam actuated rocker arm. However, with this type of arrangement a relatively large force is required to overcome the spring and lift the valve off its seat.
In order to overcome this drawback, it has been proposed to use a rocker arm arrangement which is basically comprised of a pair of rocker arms. These rocker arms are arranged in a manner which resembles a pair of scissors. One of the arms is used to lift the valve while the other is used to close the same.
Examples of such arrangements can be found in JU-A-61-6611, JP-A-60-32910, JP-A-60-39211, JU-B-53-51928 and JP-A-60-3412.
However, the arrangements disclosed in JP-A-60-3412, JP-A-60-32910 and JP-A-60-39211 for example, it is necessary to provide screw threads on the upper ends of the valve steps in order to enable the suitable retainer element to be connected thereto and to facilitate clearance adjustment to be carried out. The provision of the threads on the upper end portion of the valve stem weakens the same and invites the formation of fractures and the like which leads to breakages and/or similar malfunctions. In addition to this the number of parts which are required is increased and also increases the cost and the mass of the moving elements.
With the above type of arrangement, clearance settings must be carried out while the engine is cold and in a manner which anticipates the expansion which results from the engine warming up and which is therefore is a compromise which tends to provide excessive clearances during cold starts.
On the other hand, JU-A-61-6611 is such as to feature a simplified construction and the provision of a spring which tends to reduce the clearance between the closure arm and the lower face of retainer which is pinned to the upper end of the poppet valve stem. However, the clearance between the lift arm and the top of the valve stem is determined in accordance with the setting of a clearance adjust screw. Therefore, this arrangement also suffers from the hot/cold clearance compromise problem.
The arrangement disclosed in JU-B-53-51928 is such as to require two rocker arm support shafts, a retainer in which a spring is disposed and a clearance adjust screws on the arms which lift the valve. The clearance between the arms which close the valve are adjusted by mounting the arms on eccentric portions of the shaft on which they are pivotally supported and selectively rotating the shaft to a position wherein the clearance is set a desired value. Again the hot/cold clearance compromise problem is encountered while the need for two separate rocker arm shafts tends to increase the construction complexity and bulk of the cylinder head.